This invention relates to a method for recovering oil from an underground reservoir by caustic flooding. More particularly, the invention concerns the use of a lignosulfonate to raise the optimal salinity of a caustic flood to a higher salinity and increase oil recovery.
One method of waterflooding for enhanced oil recovery that has see considerable research and development is caustic flooding. Alkaline or caustic flooding uses relatively low concentrations of caustic chemicals such as hydroxides or carbonates to decrease the interfacial tension between the injected fluid and the oil. A considerable number of caustic flooding methods have been tried in the field in reservoirs containing anything from light oils to tar sands.
In caustic flooding, the pH value of the injected flood water is increased by adding a caustic compound. The most common chemical choice is sodium hydroxide because of its ready availability and low cost. Other compounds which may accomplish the same result include potassium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, trisodium phosphate, ammonia, or mixtures thereof. The caustic flooding solutions can range from about 0.1 to 5% or more sodium hydroxide by weight, but typically are less than 1% in field use.
U.S. Pat. No. 3,927,716 discloses an alkaline waterflooding process in which the pH and salinity of the alkaline solution are controlled in relatively narrow ranges. By controlling the salinity of the caustic flood, a lower interfacial tension is achieved between the oil and water of the flood, resulting in a greater recovery efficiency.
Lignosulfonates have been found to have excellent properties of sacrificial agents when added to aqueous flooding systems. They are economically attractive because they are by-products of the pulp industry. Supply is plentiful and product costs are much less than the cost of surfactants employed in enhanced oil recovery floods. The use of various lignosulfonates has been disclosed extensively in the literature.
U.S. Pat. Nos. 4,157,115 and 4,271,906 disclose several methods of using aqueous solutions of lignosulfonate salts as sacrificial agents in surfactant flooding to mitigate the loss of surfactant from the flooding solution to the formation. The use of oxidized lignosulfonates is disclosed in U.S. Pat. Nos. 4,133,385 and 4,196,777. Chrome lignosulfonates as sacrificial agents are described in U.S. Pat. Nos. 4,142,582. 4,172,497 discloses the use of lignosulfonates carboxylated with chloroacetic acid and U.S. Pat. No. 4,172,498 discloses sulfomethylated lignosulfonates as sacrificial agents. U. S. Patent No. 4,479,542 describes a sacrificial after flush method employing lignosulfonates. U.S. Pat, No. 4,627,494 discloses a lignosulfonate and carbohydrate system as a sacrificial agent for aqueous surfactant flooding.
A method of using lignosulfonates to prevent the precipitation of insoluble hydroxides in alkaline flooding solutions is disclosed in U.S. Pat. No. 4,466,892. In this method, at least 1% lignosulfonate is added to the water used for an alkaline waterflood before any caustic compound is added to the water. The explained purpose of adding the lignosulfonate to the injection water is to prevent the formation and precipitation of insoluble compounds formed between divalent ions in the injection water and hydroxide or carbonate ions added by the caustic material. Thus, the patent emphasizes the importance of, and in fact requires that the lignosulfonate be added to the injection water prior to the addition of caustic.
With this use in mind one of ordinary skill in the art would not consider adding lignosulfonate to waters that lack a substantial divalent ion concentration. However, the instant invention demonstrates that lignosulfonates are beneficial to flooding efficiency where divalent ion precipitation is not a problem.